A lack of CCR5 expression is associated with resistance to HIV-1 infection. This deficiency is due to homozygosity of a 32bp deletion in the CCR5 gene (CCR5 32). Several CCR5 32 individuals lacking CCR5 expression (CCR5-/-) yet HIV-1-seropositive have been identified. How the virus overcomes the CCR5 32 protective effects have not been delineated and we have investigated this in six HIV+ CCR5-/- patients. We demonstrated that peripheral blood mononuclear cells (PBMCs) isolated from exposed/uninfected (EU) CCR5- /- individuals express a truncated CCR5 32 protein. In contrast, four exposed/infected (EI) CCR5-/- patients lack expression of the CCR5 32 protein and two others expressed low levels. Pulse-chase experiments demonstrated instability of the protein in three HIV+CCR5-/- individuals. Another two had a mutation in the promoter region that was associated with low translation of the CCR5 32 protein. The lack or low CCR5 32 protein expression was associated with increased surface CXCR4. However, our results indicate that the loss of resistance in the HIV+CCR5-/- patients cannot be solely due to their increased CXCR4 levels. Expression of recombinant CCR5 32 protein restored the protective effect in three HIV+ CCR5-/- PBMC samples but failed to restore the protective effect in samples from another three HIV+ CCR5-/- individuals. Two potential mechanisms are proposed to be investigated for the loss of CCR5 32 expression in HIV+CCR5-/- subject; 1) a trans effect caused by the infecting virus or 2) a cis host effect that ultimately resulted in the creation of new signaling pathways and the loss of the resistance phenotype. We found that envelope glycoproteins (Envs) cloned from two independent CCR5-/- patients showed expanded coreceptor usage. We hypothesize that the primary HIV-1 strains isolated from the HIV+CCR5-/- individuals have unique structural determinants that allowed them to bypass the protective effect of the CCR5 32 mutation. Rapid CD4+ T cell loss despite mild viral load is characteristic of the chronic infection of these HIV+CCR5-/- patients. The novelty of our approach lay in the unique nature of these primary isolates to infect and replicate in CCR5-/- cells in vivo. The overall goal of this proposal is to define the molecular basis for the loss of the CCR5 32 protective effect. To do this we will characterize primary HIV-1 isolates from CCR5-/- patients, evaluating Env fusogenicity, apoptosis induction ability, coreceptor recognition patterns, and we will determine the role of the novel 31 residues at the C-terminus of CCR5 32 protein. PUBLIC HEALTH RELEVANCE: This application proposes to investigate the kind of HIV viruses harbored in infected homosexual individuals. Since HIV is the virus that causes acquired immunodeficiency syndrome (AIDS), the basic knowledge of these HIV viruses will provide an important new insight into AIDS pathogenesis. This project is very relevant to public health since the results of the proposed research will provide new mechanistic approaches to the design of drugs that block HIV-1 infection.